1. Field of the invention
The present invention relates to a device for measuring ion activity for the quantitative analysis of specific ion activity (or ion concentration) contained in an aqueous liquid, particularly a body fluid such as blood, urine or saliva, utilizing potentiometry.
2. Description of prior art
There has been known a device for measuring specific ion activity contained in a liquid sample of an aqueous liquid (e.g., tap water, river water, sewage or industrial drainage) and a body fluid (e.g., blood such as whole blood, plasma and serum, urine, or saliva) by using an ion selective electrode sheet.
A reference liquid and a liquid sample are spotted onto surfaces of ion-selective membranes, respectively, which are arranged on the top of at least a pair of ion-selective electrode sheets, and then potential difference between the ion-selective electrodes is measured under the condition that both liquids are electrically connected to each other by means of a bridge, so as to determine the ion activity of the liquid sample.
Examples of the ion activity measuring device employable in the method are described, for instance, in Japanese Patent Provisional Publications No. 52(1977)-142588 (corresponding to U.S. Pat. No. 4,053,381), No. 56(1981)-6148 (corresponding to U.S. Pat. No. 4,273,638) and No. 58-211648 (corresponding to U.S. Pat. No. 4,437,970).
In these devices, a pair of ion-selective electrode sheets are arranged in such a manner that ion-selective membranes are positioned on the upper side, and on the frame covering the ion-selective electrode are provided liquid receiving openings (openings for allowing introduction of a reference liquid and a liquid sample) A reference liquid and a liquid sample are spotted onto the ion-selective membranes through the liquid receiving openings using a pipet, etc., and a potential difference between both ion-selective electrodes is measured, thereby measuring the ion activity.
The device described in Japanese Patent Provisional Publication No. 58(1983)-211648 comprises plural pairs of ion-selective electrodes, which can determine ion activity of plural kinds of ions by only once spotting of a reference liquid and a liquid sample thereonto. The ion activity measuring device described in the above publication is characterized in that an ion-selective electrode sheet is arranged in such a manner that the ion-selective membrane is provided on the upper side An ion-selective electrode sheet generally comprises an electroconductive layer, a layer of water-insoluble salt, a layer of a water-soluble salt (sometimes not provided) and an ion-selective membrane laminated in this order on an insulating support. At the exposed portion (a portion exposed by necessity) of the electroconductive layer is provided an electricity-connecting region. The electricity-connecting region is brought into contact with a probe of a potential difference measuring device so as to determine the potential difference When the ion-selective membrane is placed on the upper side as disclosed in the above publication, the probe for measuring the potential difference and the electric wiring connected to the probe naturally are provided on the upper side of the ion-selective electrode sheet. The potential difference to be measured is so little that a head amplifier is desired to be arranged near the probe. Therefore, the head amplifier is required to be provided in the vicinity of the ion-selective electrode sheet. These electric parts occupy the upper space of the ion activity measuring device. On the other hand, a certain space at the upper portion of the ion activity measuring device is required for introduction of a reference liquid or a liquid sample by using a pipet. Therefore, the electric parts should be so arranged as not to disturb introduction of liquids. To meet with the requirement, the size of an automatic measuring device wherein an ion activity measuring device is automatically supplied so as to continuously measuring a plurality of samples becomes large.
In case that the electricity-connecting region is exposed with facing upward, the region and the vicinity of the region are apt to be stained with a liquid sample or a reference liquid supplied from the upper direction, thereby causing insufficient connection or insulation. Accordingly, there sometimes occurs errors in measurement of potential difference.
In measurement of ion activity, the area of an active surface of an ion-selective electrode (a surface of an ion-selective membrane) where liquids to be measured (a liquid sample and a reference liquid) are brought into contact is required to be unchanged within a certain range for securing the accuracy in measurement. In the ion-selective electrode sheet wherein a liquid sample is supplied to the upper surface of the sheet, means for defining the area where the liquid is supplied, for example, a mask provided with an opening is required In mass production of ion activity measuring devices, it is complicated and inefficient to produce a mask provided with an opening having accurate size and to arrange accurately the mask on the electrode sheet.